In the process of establishing an oil or gas well, the well is typically provided with an arrangement for selectively excluding fluid communication with certain zones in the formation to avoid communication with undesirable fluids. A typical method of controlling the zones with which the well is in fluid communication is by running well casing down into the well and then sealing the annulus between the exterior of the casing and the walls of the wellbore with cement. Thereafter, the well casing and cement may be perforated at preselected locations by a perforating gun or the like to establish fluid communication with product bearing zones in the formation. The cement also prevents the fluids in adjacent zones which are otherwise sealed from the zone of interest by a fault or other geological condition from bypassing the geological seal by moving along the wellbore or well casing. However, the casing is typically subject to corrosion by contact with particular fluids in the wellbore or other material in the borehole.
It is conventional to provide corrosion protection for the casing in the wellbore by connecting the pipe to a metal plate selected for its electrochemical properties that cause the metal plate to corrode in preference to the casing. For example, magnesium is typical metal for cathodic protection of other metals such as steel. Typically, such protection is provided at the surface after the casing has been installed into the wellbore. However, the processes of corrosion and the establishment of cathodic protection for the pipe is not entirely satisfactory. There is a delay between the installation of the metal plate cathodic protection and the time it takes to protect the full casing. Thus, there is an initial period after the installation of the casing and before the cathodic protection is fully effective where portions of the casing will be subject to corrosion. If the casing were to be corroded through at any point it may compromise the seal between the casing and the formation.
Accordingly, it is an object of the present invention to provide a method and apparatus for protecting the pipe from corrosion which overcomes or avoids the above noted limitations and disadvantages of the prior art.
It is a further object of the present invention to provide a method and apparatus for protecting a pipe in a wellbore from corrosion from the time it is installed into the wellbore until supplemental corrosion protection may be provided.